Thermomechanical fatigue in lab-type lab-type Sn–Ag–Cu solder interconnections between two copper plates has been investigated under cyclic thermal loading within a number of temperature ranges. Fatigue mechanisms have been studied using optical and scanning electron microscopy. Among the various fatigue mechanisms observed the occurrence of persistent slip bands is notable. From the correlation of the microscopic observations and stress fields calculated using elastic finite element modelling, it appears that three crucial factors govern the evolution of fatigue damage: thermal mismatch between Cu and solder, intrinsic thermal mismatches caused by Sn anisotropy and the mechanical constraints posed by the Cu on the soldered joint. The stress fields resulting from these combined sources determine the location and severeness of fatigue damage in solder joints. The relative predominance of these factors is discussed.